The present invention relates to an improvement in an electric discharge lamp used as a source of fluorescent illumination.
Fluorescent lamps have recently found increasing application as replacements for incandescent light bulbs (A-Lamps) and incandescent reflector lamps (R-Lamps) by reason of their five-fold greater luminous efficiency and much longer life. Several technical improvements have combined to yield fluorescent lamps with high light output which can fit within the confines of fixtures designed to accommodate incandescent light sources. Examples include the single-ended PL lamps of Philips Lighting in which the electric discharge is in 10 millimeter diameter glass tube formed into a tight U-shaped configuration, and the double twin single-ended lamps of GTE Sylvania in which 10 millimeter diameter glass tube is formed into a joined pair of tight U-shaped configurations. Further examples of the prior art are disclosed in U.S. Pat. Nos. 4,199,708 and 4,587,453, each comprising a joined pair of U-tubes.
Another design of compact fluorescent lamp, involving forming round section glass tube into a circular annulus, has long been available as a Circline.RTM. unit.However, the lighting area of annular lamps is not efficient in comparison with the space occupied, as noted by Sugiyama et al. U.S. Pat. No. 3,191,087. Their size and shape does not often permit direct substitution for incandescent lamps in existing fixtures.
The length and cross-sectional area of the glass tube in which the electric discharge is confined are fundamental design parameters of a fluorescent lamp. The operting voltage and current of the lamp are largely determined by the length and area of the arc path. Other factors being equal, the shorter in length and larger in area the arc path, the lower the operating voltage and the higher the operating current of the lamp. The cross-section of the tube also has a strong effect on the luminous efficiency of the lamp. The narrower the tube, the better the coupling between the arc and the light-emitting phosphor coating on the inner surface of the tube, which tends to enhance the luminous efficiency of the lamp. Operation of an electric discharge lamp requires provision of current-limiting means. The long-established, low-cost method of current control is by series connection of a core-and-coil ballast. This method is most effective with lamps operating at no more than half of the supply voltage. Lamps designed for use on the 120 volt supply which is standard in the U.S.A. and in many other countries are often, accordingly, limited to a lamp voltage of 60 volts. This limitation serves to rule out for U.S. use with a core-and-coil ballast some designs of compact fluorescent lamp which have considerable utility but which operate at more than 60 volts across the lamp. Some relief of this design limitation is in sight, by employing solid state ballasts to control the electrical starting and operating conditions of fluorescent lamps, by which means the lamp can operate at a voltage higher than 50% of the voltage of the source of supply. However, solid state ballasts are at the present time more expensive and less reliable than core-and-coil ballasts.
In summary, presently available fluorescent lamps of annular, single U-shaped or double U-shaped configuration cannot in the majority of cases be directly substituted for incandescent light bulbs and reflector lamps in existing fixtures by reason of inappropriate shape, larger size per unit of light output and a lesser level of illumination than in many widely used incandescents.